Creating Your Colorado-friendly Garden: 2.a - Site Assessment: Soils

Soil Survey Map: Mother Nature's Montrose Garden

In this post, we continue to explore the steps in creating a Colorado-friendly garden. For a complete list of posts in this series see: https://mothernaturesmontrosegarden.blogspot.com/p/creating-your-colorado-friendly-garden.html

As we discussed in our introduction (July, 2019), the ‘Colorado-friendly Garden’ involves designing gardens around existing conditions – what we like to think of as a site’s ‘natural assets’. While this series is aimed primarily at Colorado gardeners, the principles are applicable wherever you live; they are, quite simply, the principles behind sustainable gardening. The key point is this: get to know your garden’s ‘natural assets’, then choose plants and hardscape appropriate for these assets. The result is a healthier, more sustainable and often more interesting landscape.

Throughout this series, you’ll be developing maps, lists and worksheets specific for your garden. We suggest you create a Garden Planning Notebook to keep all these materials together in one place.  You’ll also want to include pictures of inspirational gardens, ideas about plants and hardscape – anything else that you may want to include in your final garden plan.

Soil is a complex system; it's characteristics

 have a large impact on plant health

Site physical conditions will be different for every garden; that’s why each gardener needs to discover his/her site’s own assets, before considering a garden design. Site characteristics include such things as soil type and drainage, sun and shade patterns, wind, temperature and even views (good or bad). You need to assess these site assets before you plan your garden, so you can take advantage of them. This post and the next will walk you through the process.

Several physical characteristics have the most effect on plant suitability and plant health. These include: soil texture and drainage; precipitation and precipitation patterns; the amount of light (sun and shade); temperature; and wind. We discuss each of these in a separate section. You will find there’s lots to learn about your garden – information that will actually make your gardening easier.

By the end of this exercise (end of September) you will know a lot more about your site. You will carefully observe conditions in your yard; you’ll also conduct some simple tests. We suggest you complete several summary sheets with key information about your site. The summary sheets are available at:

• http://www.slideshare.net/cvadheim/soil-characteristics-of-your-site-summary (soils summary).
• http://www.slideshare.net/cvadheim/physical-characteristics-of-your-site-summary (site summary)

You will also create several maps that will help you design your garden on a solid basis: a soils map (if needed); a sun and shade map; and a map (or two) with other physical features. If you don’t yet have a good base map for your garden, see our July (2019) postings on Creating a Base Map.

Simple soils tests help you understand

 your soil's characteristics

Soil type and drainage.

Soil is truly a garden’s foundation.  Plants obtain water and many nutrients from the soil – and interact with soil organisms.  Soil conditions may also limit the types of plants you can grow successfully.  The more you know about the characteristics of your soil, the better you can match plants and watering methods to your unique soil conditions.

A well-developed soil is a complex ecosystem.  When people talk about soils, they’re usually referring to the inorganic part of the soils – the sand, silt and clay particles.  But a good soil contains much more.  It has pores which hold vital air and water.  It contains decomposing organic matter (old roots; decayed leaves; etc.) that provide plant nutrients and help retain soil moisture.  And it’s home to a whole world of soil organisms, ranging in size from gophers and earthworms to microscopic bacteria and fungi.  In fact, a good soil is literally teeming with life!

Soils are characterized by the parent material (rock) from which they were formed and the conditions of their formation. In western Colorado, most of our soils were either deposited by ancient shallow seas and/or from stream erosion from local mountains.  Many local soils, particularly in agricultural regions are loams and clay loams.  But there are regional differences that may affect your garden.  If you’d like to learn more about the original (native) soils for your local area we suggest the following resources:

• Soil Web (soil maps/information for specific geographic locations.  This is an easy-to-use resource): https://casoilresource.lawr.ucdavis.edu/soilweb-apps
• Natural Resources Conservation Service, Colorado (a bit less intuitive to use; good on-line resources): https://www.nrcs.usda.gov/wps/portal/nrcs/detail/co/soils/surveys/?cid=nrcs144p2_063022
• Colorado Soil Conservation Districts (some districts have useful on-line resources; all have helpful people): http://www.coloradoacd.org/conservation-districts.html

Urban soils – including those in our gardens - are more complex than native soils. In fact, urban soils are among the most difficult to classify – and work with – of any soils world-wide.  Like all soils, urban soils are composed of a parent rock material that has weathered over time to produce soil.  But it’s the human effects that make urban soils so ‘difficult’.  Soil scientists are just beginning to get a true understanding of the differences between urban and native soils – and their effects on plant growth.  To learn more about urban/suburban soils, see: http://mother-natures-backyard.blogspot.com/2012/09/getting-to-know-your-gardens-soil-urban.html.

After precipitation, soil type and drainage have the most effect on plants. Two key characteristics that effect plant health are soil drainage and soil fertility.  Interestingly, both characteristics are influenced by the types/sizes of the particles that make up a soil – the physical characteristics of the soil.  You may suspect that you have a loam or clay soil, since both are common in our area.  In general, the following are true:

• Sandy soils - have larger soil particles that allow them to drain well.  They also dry out more quickly and tend to leach out their nutrients.  If your soil is sandy, you’ll need to water more frequently in summer.
• Clay soils - have smaller soil particles that retain water and soil nutrients better.  They also tend to drain more slowly, which can create problems for some water-wise plants.  If your soil is clay, you’ll need to water less frequently to avoid over-watering.
• Loam soils – have a balance of sand, clay and intermediate size (silt) particles.   They are best for retaining water and nutrients – in fact, they are ideal soils.  Loam soils are more common than you think in our area.

You may already think you know your soil type. But until you test, you won’t really know.  Here are some Simple Soil Tests to help you get to know your garden soil better:  http://mother-natures-backyard.blogspot.com/2012/10/getting-to-know-your-gardens-soil.html 

Much soil profile data is available on-line. 

But conditions in your garden may differ, due to past

 usage and gardening practices

Soil depth

Some poorly drained soils have a dense layer that keeps water from draining.  This may be a true ‘hardpan’ clay layer, a rock layer or just a compacted zone.  You can often detect a dense layer by digging down until you cannot dig further.  A depth of less than 20 inches means you have a shallow soil.  You may want to break up the impervious layer (or drill holes through it) to improve drainage.

Soil pH

Soil pH influences the availability of soil nutrients to your plants.  Most local garden soils have a pH from about 7.0 to 8.0 or higher.  An ideal soil for many plants (including vegetables) is around 7.0. But there are many native plants that like a soil pH between 7.5 and 8.0 (‘basic’ soils).  Some local native plants even thrive in very alkali soils (pH > 8.0).  If you’re having difficulty getting plants to grow – or if you want to plant ‘acid-loving’ plants - you can test your soil pH with a simple test kit (available at most garden centers and plant nurseries). There are also electronic meters that measure pH; we have one that also measures soil moisture and available light (see above)

Soil basic nutrients

Many Colorado native plants have low fertilizer needs; other plants (including vegetables and some non-native plants) have higher nutrient needs.  Simple test kits that indicate levels of the basic nutrients (nitrogen; potassium; phosphorus) are available at most garden centers.  These kits are inexpensive and easy to use - just follow the directions on the kit. Most kits indicate whether the levels of nitrogen, potassium and phosphorus are low, adequate or high.  That’s usually good enough for most gardens.  For more on soils testing, see references 1-3, below.  

If you want to learn more about your soil’s nutrients you can send a soil sample to a soil testing lab.  These labs can measure additional nutrients - and with greater precision than simple test kits.  One of the most reasonably priced labs is at Colorado State University (http://www.soiltestinglab.colostate.edu/), where a basic soils analysis is $35.00.  Another good lab which offers garden soils analysis is Midwest Laboratories (https://midwestlabs.com/consumer-estore-2/consumer-estore-garden/).  The University of Massachusetts Center for Agriculture also offers basic soils testing. (http://soiltest.umass.edu/).   Visit the websites to learn more about the tests available, prices, etc.

Testing soils: Mother Nature's Montrose Garden

We’ve learned a lot about the soils in Mother Nature’s Montrose Garden this month. After testing several places in the garden we know that:

• Our soil is clay-loam (Mack clay loam) throughout. The soils are primarily stream terrace soils.
• Our soils are deep, with no hardpan
• Our soils are well-drained
• Our pH in most of the garden is 7.8 to 8.0
• Our soils have average levels of key soil nutrients (N/P/K)

After you complete your own soil and drainage evaluation, we suggest you complete the soils summary sheet (http://www.slideshare.net/cvadheim/soil-characteristics-of-your-site-summary) and place it in your Garden Planning Notebook.  And we hope you join us later in the month, when we consider other physical features that influence your garden.

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1. http://websoilsurvey.nrcs.usda.gov/app/
2. http://www.rain.org/global-garden/soil-types-and-testing.htm
3. ftp://ftp-fc.sc.egov.usda.gov/NSSC/Educational_Resources/concepts2.pdf

We welcome your comments (below).  You can also send your questions to: monaturesmontrosegarden@gmail.com